Modern medical imaging methods permit physicians to more accurately diagnose and treat a wide variety of disorders. Such imaging methods are based on various technologies including acoustic waves (ultrasound), radioactive decay (positron emission tomography), and nuclear magnetic resonance (magnetic resonance imaging). Each of these imaging techniques has its own characteristic advantages and disadvantages, but medical researchers, physicians and other practitioners continue to seek higher resolution, more reliable, less invasive, and more easily interpretable imaging systems and methods in many applications. For example, coronary magnetic resonance angiography (MRA) has been used in the assessment of coronary disease. Unfortunately, the low signal-to-noise ratio (SNR) obtained at applied fields of about 1.5 T can limit the application of this technique for distal and branching vessels. Application of higher magnetic fields can improve SNR but higher magnetic fields are associated with undesirable changes in off-resonance susceptibilities, magnetic field inhomogeneities, and increased specific absorption rate (SAR).
Magnetic resonance (MR) imaging systems generally use a static magnetic field (B0) and a radio frequency magnetic field (B1) to produce images. Unfortunately, these magnetic fields cannot be controlled with arbitrary precision, and MR signals and images can be degraded by imperfections such as non-uniformities in these magnetic fields. All T2 prep sequences consist of an initial 90° pulse to convert a substantial part of the longitudinal magnetization in the image field of view to transverse magnetization, a combination of delays and RF pulses designed to refocus this transverse magnetization after some signal decrease through T2 relaxation during these pulses and delays, followed by a final 90° pulse to return a substantial part of the refocused magnetization to longitudinal magnetization. The T2 relaxation between the two 90° pulses provides the desired alteration of contrast between components of the sample with different T2 relaxation rates. Conventional T2 preparation (T2 prep) sequences have been designed to be robust to flow as well as to inhomogeneites in both B0 and B1. Such sequences use opposing pairs of so-called Malcom-Levitt (MLEV) pulses that can compensate pulse shape imperfections in the RF magnetic field B1. Two representative sequences of such MLEV weighted composite T2 prep sequences are shown in FIGS. 1A-1B. Pulses indicated as 180x0 are composite pulses, each consisting of a 90°x 180°y 90°x pulse sequence. Such MLEV weighted composite pulses can compensate some imperfections in B1, with larger numbers of such pulses providing increased compensation. However, increasing the number of MLEV pulses results in an increase in specific absorption rate (SAR), thus limiting the use of large numbers of MLEV pulses, especially at high B0. Thus, MLEV pulse based T2 prep is unsatisfactory in many applications.
Three dimensional (3D) MRA can be used to image the tortuous path of the coronary artery tree with improved SNR relative to two dimensional MRCA. Unfortunately, 3D coronary MRA images have a low contrast between coronary blood and myocardium. Image contrast can be enhanced with contrast agents or non-endogenous magnetization sequences. T2 prep contrast enhancement can be used to increase contrast between blood in coronary arteries and surrounding tissues based on an applied T2 weighting. However, T2 prep is associated with imaging artifacts that limit clinical utility, and can be associated with unacceptable SAR. Accordingly, improved methods and apparatus are needed, particularly for T2-weighted imaging.